The finite reserves of fossil fuels and ever-increasing pressure on reducing greenhouse gas emission have generated an urgent need for alternative sources of energy. Wastewater treatment accounts for about 3% of electrical energy consumed in the U.S. and other developed countries. Wastewater is estimated to contain as much as 9.3 times the amount of energy currently consumed to treat the water in a modern wastewater treatment plant. Microbial fuel cell (MFC) technology, which uses microorganisms to catalyze the direct generation of electricity from biodegradable organic matter, provides a completely new approach for energy generation from wastewater while simultaneously treating wastewater. MFC technology holds great promise in converting wastewater treatment from an energy consuming process to a net energy producing process, thus drastically enhancing energy sustainability for wastewater treatment and reuse.
A single chamber air-cathode microbial fuel cell (MFC) provides great advantages over a two chamber system for many practical applications because 1) passive air can be used thus no aeration is needed, 2) no recycle or chemical regeneration of catholyte is required, thus the operation is simplified, and 3) smaller cell volume, thus higher volumetric power density, is easily achieved. Furthermore, air-cathode MFCs that lack a proton exchange membrane (PEM) hold great promise due to their low cost, simple configuration, and relatively high power density. Membrane-free MFCs, however, present two major challenges: (1) coulombic efficiency is much lower than that of MFCs containing a membrane when a mixed culture is used due to the consumption of substrate by oxygen diffused through the cathode; and (2) the anode and cathode distance in a membrane-free MFC is limited to a certain range (about 1-2 cm) due to the potential negative effect of oxygen on the activity of the anaerobic bacteria on the anode and the risk of short circuit. This relatively large electrode spacing not only increases the internal resistance, but also limits the volumetric power density.
Another limitation of traditional MFCs is that the voltage output of a single MFC is normally less than 0.8 V, and often less than 0.3 V at maximum power output. Such a low voltage output greatly limits the application of MFCs. In traditional methods, voltage output is increased by serially connecting several MFCs together with a conductor or current collector. However, these serial connections often lead to problems, such as voltage reversal and voltage crossover, which greatly reduce the overall performance of the MFC stack. Another solution used in the art is to use DC/DC converter to boost voltage. However, such methods not only increase the complexity, and thus lower the reliability, of a MFC stack, but also reduce the overall efficiency as a significant portion of energy is lost during the conversion process. Furthermore, a current collector is still needed for large MFC stacks, which not only increases the total cost and size of an MFC stack, but also decreases the reliability and lifetime of the MFC.